Sorption of uranyl ions on hydrous silicon dioxide

Sorption of uranyl ions on SiO2.χH2O (silica gel) is investigated in absence and in presence of carbonate as function of pH. The curves obtained are very similar to those observed for sorption of uranyl ion on TiO2.χH2O, indicating the dominating influence of the uranium species in solution. Between pH 2 and 5 the sorption ratio Rs increases with hydrolysis of uranyl ions (formation of UO2OH+), around pH 7 it is nearly independent of pH, and at higher pH it decreases again. The equilibrium constants are calculated for these ranges. In presence of carbonate Rs decreases drastically above pH 6, due to the formation of carbonato complexes in solution. Sorption of uranyl ions on SiO2.χH2O, on TiO2.χH2O, and on cryst. SiO2 and Al2O3 is compared. The problems of 'surface complexation' modelling are discussed. (orig.)